1. Field of the Invention
The present invention relates to a copy protection method for record carriers, i.e. storage media, which store digital data as an asynchronous signal, e.g. Compact Discs (CDs) and Digital Versatile Discs (DVDs) including all existing or future formats of CD and DVD or other Optical Storage Media which work according to a similar concept.
In particular, the copy protection method according to the present invention allows to make a physical copy or creating a complete data image of the media content, but the physical copy itself is not accessible anymore or the access is disturbed or that the error rate is increased dramatically so that e.g. listening to the music content of a copied audio CD is not entertaining anymore.
2. Discussion of the Background
Optical storage discs with information stored on one or both sides have come to be used for a variety of purposes, most notably in the music, games, video, and computer industry. Digital information is stored on the optical storage media in the form of pits arranged along circular, concentric tracks on one or on both sides of the disc. The track is typically read from the inside out, but may also be read from outside in, as it is already used for some optical storage media.
The data itself on the track is subdivided into frames, each equal in length, containing equal amounts of information. Each frame has a dedicated layout depending on the type of optical storage media (CD, DVD). Such a frame always contains the user data symbols itself but may also contain data for synchronization, merging data between data symbols and error correction.
The signal on a optical storage medium itself is asynchronous, which means that in the decoding process synchronization, timing information, parity data or other data has to be spied out from the signal.
Due to the nature of such storage media copies can be made easily. To cope with this situation, there exist various copy protection schemes which try to prevent unauthorized access with the help of keys or passwords stored on the respective storage medium itself. Further, there exist methods which secure an optical disc with the help of stored invalid information which gets repaired during the copy process so that an application program can determine whether it is stored on an original or on a copy based on the presence or absence of this invalid information.
Further, U.S. Pat. No. 6,005,839 describes a copy protection scheme for optical record carriers, in particular DVDs, according to which direct or dump copying is prohibited by inserting a synchronization pattern at an irregular position different from the regular position or by inserting a synchronization pattern different to the standard synchronization pattern at the regular position of certain areas or sectors of the optical record carrier. To assure an undisturbed reproduction of such a record carrier the reproducing obstructing area created with the misleading synchronization patterns is skipped by jumping based on position designating information (designating the reproducing obstructing area) which is recorded on the record carrier, preferably in the Table Of Contents (TOC) area.
Further, according to WO 01/78074 which content is herewith incorporated into this specification, additionally generated data which confuses a reading device gets written to at least one specific portion of the record carrier, e.g. the beginning of a sequence, so that a general purpose reading device which can access record carriers of different formats, e.g. a CD-ROM, which accesses said record carrier judges said record carrier to be inaccessible, i.e. that a record carrier according to that teaching comprises at least one specific portion with such additionally generated data.
Therewith, according to that teaching such additionally generated data which confuses a reading device is inserted in such areas which are not accessed by a reading device designed for that type of record carriers, e.g. a normal audio CD player for audio CDs, but which are always accessed by a general purpose reading device, e.g. a CD-ROM reading device, so that these reading devices will abort the reading or access of the record carrier.
However, due to the fact that such record carriers are generally not accessible by e.g. CD-ROM reading devices, users who want to also use these record carriers on their computers are not satisfied.
U.S. Pat. No. 5,699,434 which content is herewith included by reference into this specification discloses a method of inhibiting copying of digital data according to which a sequence of symbols is added to the original data, wherein the sequence of symbols is selected to encode into channel bits having a large accumulated digital sum variance (DSV). The sequence of symbols is then encoded by a special encoder that generates special channel bits that do not have a large accumulated digital sum variance so that no playback problems will occur. However, in case of a normal encoding of the accessed data including the added sequence of symbols for the purpose of copying or further recording, i.e. a re-encoding, channel bits having a large accumulated digital sum variance will be likely generated. The generated digital sum values can be used to encode additional data, e.g. by their sign, so that other encoders will encode the same original data into channel bits that do not include this additional data which might be used for decrypting, descrambling or otherwise modifying the original data. Further, it is stated that other encoders might also encode the same original data in channel bits that cannot be recovered.
However, according to this copy protection method always a special encoder is necessary adapted to encode the selected sequence of symbols which would normally encode into channel bits having an accumulated digital sum variance that exceeds a predetermined limit non-optimally into channel bits having an accumulated digital sum variance that does not exceed the predetermined limit. Due to the needed special design such an encoding can become costly.